Homopolymers of THF, also known as polytetramethylene ether glycols (PTMEG), are well known for use in spandex, polyurethanes and other elastomers. These homopolymers impart superior mechanical and dynamic properties to polyurethane elastomers, fibers and other forms of final products. As discussed in U.S. Pat. No. 4,120,903, the polymerization process of utilizing tetrahydrofuran (THF) to manufacture polytetramethylene etherglycol (PTMEG) by passing through the intermediate PTMEA (i.e., PTMEG diacetate) has been commercially practiced since about 1997. The process involves a ring-opening of THF using perfluorosulfonic acid ionomer resin as the first step in the production of the PTMEA. The most commonly known process to convert PTMEA to PTMEG is by a conventional transesterification using an alkali metal catalyst, such as sodium methylate. This method results in a residual catalyst which needs to be removed from the PTMEG product.
There are many known processes for removing the remaining alkali metal catalyst from the PTMEG product after the transesterification step. Some of these known processes are disclosed in U.S. Pat. Nos. 4,137,396, 4,985,551, 4,460,796, 4,306,943 and 6,037,381. U.S. Pat. No. 5,410,093, herein incorporated in its entirety by reference, relates to a method wherein the alkali metal catalyst is neutralized in an aqueous media in the presence of an excess of magnesium sulfate. The inorganic co-products of this neutralization can include sodium sulfate and magnesium hydroxide. The various inorganic solids present in PTMEG are then separated in a chamber plate filter press operation. The filter press utilizes a filter cloth to aid in the removal of solids from the filtrate.
Filtration of solutions containing gelatinous solids is extremely difficult and slow through normal filter cloths and generally blinding of the cloth occurs with consequent cessation of flow through the filter. Normal procedure for the improvement in filtration rate involves the use of a filter aid which protects the filter cloth and retains an open structure allowing sufficient flow of filtrate and also clarification of the suspended solids by entrapment in the porous cake. Therefore, it was observed that the filtration of the alkali metal catalyst from the polymer required a filter aid to be successful. The filter aid is required for pre-coating the filter press cloth support media and further provided as a continuous feed to be mixed with the PTMEG suspension containing the variety of inorganic salts. It is widely known to use diatomaceous earth (DE) based filter aids to aid in the filtration process. However, the inventors of the present application have found that that the use of DE, which contains transition metal oxides, as a filter aid, has resulted in the leaching of transition metals catalyst species capable of possibly promoting transester formation and subsequent quality problems for downstream users of the PTMEG.
Therefore, there is a need for a filter aid that can be used in the filtration process to remove an alkali metal catalyst from a transesterification process to form a polyether polyol that does not result in transition metals catalyst species being leached in the polyol product.